1. Technical Field
The present invention relates to a surface light source device and a frame used therein. For example, the present invention relates to a surface light source device that is used as a backlight illuminating a liquid crystal panel, and to a frame that surrounds a lightguide plate and the like in the surface light source device.
2. Related Art
In a liquid crystal display unit of a mobile phone, a surface light source device (backlight) for illumination is disposed behind a liquid crystal panel. The surface light source device is configured while a lightguide plate and a light source are accommodated in a frame, light output from the light source is incident to the lightguide plate from an end face (light incident end face) of the lightguide plate, and the light is output from a light output surface of the lightguide plate while spread over the whole lightguide plate.
[Surface Light Source Device with White Frame]
Generally, the frame of the surface light source device is molded using a white resin. This is because, when the white-resin frame is used, the light that leaks from an outer peripheral surface of the lightguide plate can be reflected by the frame and incident to the lightguide plate again, and luminance of the light output surface can be improved.
However, when the white-resin frame is used, the light that leaks from the lightguide plate cannot completely be reflected by the frame, and part of the light is transmitted through the frame to leak out. In particular, recently a display screen is increased in size while the liquid crystal display unit is thinned. For example, in a large-screen liquid crystal display apparatus provided with a touch panel, a width of the display screen is substantially equal to a width of the mobile phone. An area of an effective emission region is enlarged while external dimensions of the surface light source device are restricted. As a result, it is inevitable to narrow the frame of the surface light source device, and a thickness (width) of the frame is thinned more and more (for example, thickness of frame ranges about 1 mm to about 2 mm).
Consequently, an amount of light that is transmitted through the frame to leak out is gradually increased. For example, when the amount of light that is transmitted through the frame to leak from the surrounding of the surface light source device is increased, an outer peripheral surface of the surface light source device is brightened to degrade visual quality of a liquid crystal display unit. When a photosensor or a light receiving portion for infrared communication is disposed near the surface light source device incorporated in the mobile phone, the light that leaks from the outer peripheral surface of the surface light source device possibly causes a malfunction of the photosensor or the light receiving portion.
[Surface Light Source Device with Black Frame]
Therefore, the frame is molded using a black resin, and the light that leaks from the outer peripheral surface of the lightguide plate is absorbed by the frame to decrease the light that leaks from the outer peripheral surface of the surface light source device. For example, Japanese Unexamined Patent Publication No. 11-52140 (FIG. 9) discloses a surface light source device in which the frame is molded using the black resin.
As illustrated in FIGS. 1A, and 1B, in a surface light source device 11, a reflecting sheet 12 is bonded to a lower surface of a rectangular frame 13 made of a black resin, thereby covering the lower surface of the frame 13 with the reflecting sheet 12. Plural diffusion sheets and optical sheets 17 are accommodated in the frame 13 while overlapped on a surface (upper surface) on a light output side of a lightguide plate 14. Plural light sources 16 mounted on a lower surface of a flexible printed board 15 are also accommodated in the frame 13, and each light source 16 is disposed opposite an end face of the lightguide plate 14. A frame-shaped light shielding sheet 18 made of a black resin sheet is bonded to the upper surface of the frame 13, whereby the edge of the lightguide plate 14 is covered with the light shielding sheet 18 while the lightguide plate 14 is retained between the light shielding sheet 18 and the reflecting sheet 12.
As illustrated by a solid-line arrow in FIGS. 1A and 1B, when the frame 13 is molded using the black resin, because the light that leaks from a side surface of the lightguide plate 14 is absorbed by the frame 13 made of the black resin, the light hardly leaks from the outer peripheral surface to the outside of the surface light source device 11.
However, in the surface light source device 11, because the light that leaks from the side surface of the lightguide plate 14 is absorbed by the frame 13 to eliminate the reflected light illustrated by a broken-line arrow in FIGS. 1A and 1B, the use efficiency of the light is degraded, the amount of light output from an emission surface of the surface light source device 11 is decreased to lower the emission luminance, and a variation in luminance is easily generated.
An experiment and a simulation report that, in the case in which the frame is molded using the black resin, the emission luminance is decreased by about 10% compared with the case in which the frame is molded using the white resin. The decrease in emission luminance is confirmed by the following experiment and simulation. (1) In the surface light source device in which the frame having four sides made of the white resin was used, it was assumed that the average luminance was set to 100% when the luminance was measured at 25 points (the number of measuring points) in the emission surface. In the surface light source device that has the same structure except that the frame having four sides made of the black resin was used, the average luminance was obtained when the luminance was measured at 25 points. As a result, the average luminance of 92 to 93% was obtained in the case in which the black-resin frame is used. (2) In the surface light source device in which the frame having four sides made of the white resin was used, it was assumed that the average luminance was set to 100% when the luminance was measured at 289 points in the emission surface. In the surface light source device that has the same structure except that the frame having four sides made of the black resin was used, the average luminance was obtained when the luminance was measured at 289 points. As a result, the average luminance of 90 to 92% was obtained in the case in which the black-resin frame is used. (3) In the surface light source device in which the frame having four sides made of the white resin was used, it was assumed that the average luminance was set to 100% when the luminance was simulated at 25 points in the emission surface. In the surface light source device that has the same structure except that the frame having four sides made of the black resin was used, the average luminance was obtained when the luminance was simulated at 25 points by simulation. As a result, the average luminance of 90 was obtained in the case in which the black-resin frame is used. Therefore, in the case in which the black-resin frame is used, it is found that the black-resin frame absorbs about 10% of the amount of light output from the emission surface in the surface light source device in which the white-resin frame is used.